1. Field of the Invention
The present invention relates to a semiconductor optical device, and more particularly to a semiconductor optical transmitter.
2. Description of the Related Art
As Internet communication rapidly increases, the need for improved optical communication speed and facilities also increases. Further, rapid depreciation in optical part prices has promoted the construction of a new ultra-high speed communication environment. In such an environment, technologies are capable of realizing a modulation speed of more than 10 Gbps. In addition, long-distance transmission of more than 80 km without an erbium doped fiber amplifier (EDFA) have attracted considerable attention. Advantageously, the removal of high-priced optical amplifier parts in an optical fiber transmission line provides not only a reduction in price but also in the maintenance of facilities. Further, for an ultra-high speed long-distance transmission, a high-power optical transmitter and a high-sensitivity optical detector, which can transmit optical signals, are necessary even if a loss of optical fiber exists.
FIG. 1 is an optical transmitter according to one example of the prior art. The optical transmitter 100 includes a distributed feedback laser diode (hereinafter, referred to as DFB LD) 110, an isolator (hereinafter, referred to as ISO) 120 and a Mach-Zehnder modulator (hereinafter, referred to as M-Z MOD) 130. The DFB LD 110 continuously outputs high-power lights and the M-Z MOD 130 modulates the inputted light into communication signals at high-speed. Since the DFB LD 110 easily distorts output light due to fed-back light such as reflected light, the ISO 120 is inserted between the DFB LD 110 and the M-Z MOD 130 in order to prevent the distortion. The ISO 120 passes light inputted in one direction and isolates light inputted in other direction.
FIG. 2 is an optical transmitter according to another example of the prior art. The optical transmitter 200 includes a DFB LD 210, a first and a second ISO 220 and 240, an electro-absorption modulator (hereinafter, referred to as EA MOD) 230 and a semiconductor optical amplifier (hereinafter, referred to as SOA) 250. The DFB LD 210 continuously outputs high-power light and the EA MOD 230 modulates the inputted light into communication signals at high-speed. The SOA 250 compensates for optical loss in the EA MOD 230 by amplifying and outputting the inputted light. Further, the SOA 250 partially compensates for frequency chirp that is generated in the EA MOD 230. The first ISO 220 is disposed between the DFB LD 210 and the EA MOD 230. The second ISO 240 is disposed between the EA MOD 230 and the SOA 250. Each of the first and the second ISOs 220 and 240 passes light inputted in one direction and isolates light inputted in other direction.
However, in such a conventional optical transmitter 100, since each part is expensive, the entire cost of the optical transmitter 100 becomes very expensive. Further, in order to prevent transmission light from being distorted due to reverse-direction light such as reflected lights in connecting optical elements, it is essential to employ the ISO 120. Moreover, each part has a size of several cm by several cm and optical fibers are used to connect parts with each other. Thus, the optical transmitter 100 has an increased overall size of several tens of cm by several tens of cm. To maintaining stable operation of each part, temperature must be kept constant. Power consumption of the optical transmitter 100 becomes very great since each part consumes a large amount of power.
Further, in such a conventional optical transmitter 200, as shown in FIG. 2, high-priced parts with wide amplification band and low polarization dependency must be employed. These parts are needed since the SOA 250 is affected by conditions of the wavelength of the DFB LD 210 and a polarization of light outputted in the EA MOD 230.